In television receivers having tuners whose frequency of operation depends on a voltage applied to a tuning control terminal, it is often convenient to combine the nominal tuning voltage for a given channel with an automatic frequency control (AFC) voltage in order to effect the precise frequency of operation. In conjunction with varactor tuners, U.S. Pat. No. 3,952,143, "Wideband AFC System", by Siwko describes a novel network for combining the tuning voltage with the AFC voltage in a manner that provides an AFC correction voltage range that compensates for variations in tuning voltage sensitivity. An alternate approach is described in U.S. Pat. No. 4,005,256, "AFC Circuit", by Arumugham.
Although these circuits offer significant performance improvements, interaction between the tuning voltage and the AFC voltage has been found to pose some subtle difficulties. For examaple, in order to assure the stability of the AFC system and provide the desired AFC time-constant, the AFC voltage is typically coupled through an appropriate Resistor-Capacitor (RC) network to the tuning control terminal. In a particular embodiment the AFC voltage may be resistively coupled to the tuning control terminal and the terminal in turn coupled through a relatively large valued capacitor (on the order of one microfarad) to circuit ground. The capacitor tends to interact with the tuning voltage so that some time delay is experienced between an instantaneous change in tuning voltage -- as occurs when switching from one channel to another -- and the appearance of a corresponding change in the voltage applied to the tuning control terminal. In switching from the nominal tuning voltage of an initial channel to the nominal tuning voltage of a subsequently chosen channel, the tuning voltage may temporarily take on a value corresponding to the frequency of an adjacent sound or picture carrier. As a result the AFC will have a tendency to lock onto the undesired carrier, to the exclusion of the desired signal.
In addition, transient overshoots in the tuning control voltage can cause the receiver's local oscillator to be temporarily tuned to a frequency beyond the desired frequency of operation. That is, when the tuning control voltage is switched from an existing voltage to a lower voltage, the overshoots can cause the local oscillator to be temporarily tuned to a frequency below the desired frequency; conversely, when the tuning control voltage is switched to a higher voltage, the local oscillator may be temporarily tuned to a frequency above the desired frequency. In the event that the overshoots are of sufficient amplitude either the desired channel's associated sound carrier or the upper adjacent channel's sound carrier may be brought within the pull-in range of the AFC system and false-lock may again occur.